1. Field of the Invention
The present invention relates to a method for detecting inclination and/or displacement of an imaging plane of a radiographic image detector during recording of radiographic image information on the detector, and a device for carrying out the method.
The present invention further relates to a method for applying, to image data representing a radiographic image of a subject acquired using the radiographic image detector, correction to eliminate misalignment along a joint line in a subject image which misalignment is generated when the subject image is formed by combining the image data, and a device for carrying out the method.
2. Description of the Related Art
A panel-shaped radiographic image detector has conventionally been used in practice, as described, for example, in Japanese Unexamined Patent Publication No. 2006-156555 (hereinafter ref erred to as patent document 1). The radiographic image detector includes an imaging plane having a two-dimensional matrix of pixel sections, and when radiation which carries image information is applied, each pixel section stores an electric charge depending on the amount of the radiation. Then, the radiographic image detector outputs image data representing the image information through a reading operation. In general, each pixel section includes: a charge generation layer for generating an electric charge when exposed to radiation; a voltage application electrode for applying a voltage to the charge generation layer; a charge collection electrode for collecting the electric charge generated at the charge generation layer; and a switching element for reading out the electric charge collected by the charge collection electrode, which are formed using, for example, a TFT (thin film transistor) active matrix array.
The above-described panel-shaped radiographic image detector typically has a quadrangular shape, i.e., a rectangular or square shape, and is widely used to record image information carried by radiation which has been transmitted through the subject and applied to the radiographic image detector. The radiographic image detector having the quadrangular shape may sometimes be used to record a long image representing a long portion, such as the entire spine, of the subject, such as a human body. In this case, the radiographic image detector is shifted along a predetermined axis of shift so that each time the radiographic image detector receives radiation transmitted through a different portion of the same subject.
When the panel-shaped radiographic image detector is used in this manner, the reading operation is carried out each time the radiation is applied (each time a radiographic image is recorded) to acquire image data representing a radiographic image during each reading operation. Subsequently, the sets of image data are combined to acquire image data representing the long portion of the subject. The method to acquire image data representing a long radiographic image in this manner is described, for example, in Japanese Unexamined Patent Publication No. 11(1999)-244270 (hereinafter referred to as patent document 2), in which a cassette containing a phosphorescent screen is used as an example.
When the radiographic images are combined as described above, misalignment along the joint line may be observed in the combined image due to inclination of the imaging plane of the panel-shaped radiographic image detector. There are several types of “inclination of the imaging plane” causing this problem. Now, the types of inclination are described in detail with reference to FIGS. 10 and 11.
First, FIG. 10 schematically shows, at “a”, a side view of a system for recording (imaging) a radiographic image, which includes a radiation source 100, a stand 101 for guiding a quadrangular panel-shaped radiographic image detector D when it is shifted, and an imaging plane 102 in the radiographic image detector D. In this example, a grid 103 is recorded as the subject for the convenience of explanation of the problem. That is, radiation 104 emitted from the radiation source 100 and transmitted through the grid 103 is applied to the imaging plane 102 of the radiographic image detector D.
In this case, the quadrangular panel-shaped radiographic image detector D is orientated such that the panel surface and one side of the panel is parallel to a direction in which the stand 101 extends (the direction of arrow H), and is to be shifted in the direction of arrow H. That is, in this case, the direction of arrow H is the axis of shift. Then, first and second radiographic imaging operations are carried out by applying the radiation 104 transmitted through the grid 103 to the radiographic image detector D, which is stationary before and after being shifted.
One problem here is that the imaging plane 102 (and thus the two-dimensional matrix of pixel sections forming the imaging plane) may be inclined by an angle α with respect to the surface of the panel due to assembly error, or the like, of the radiographic image detector D. In such a case, radiographic images of the grid 103 imaged through the first and second application of the radiation are distorted, as shown at “b” and “c” in FIG. 10. That is, when the first and second recorded images are joined at an area in the vicinity of the lower edge of the first image and an area in the vicinity of the upper edge of the second image, the transverse length of the subject is different between theses areas, and misalignment is generated along the joint line.
It should be noted that, in this case, with the radiographic image detector D being set as described above, the inclination angle α of the imaging plane 102 relative to the panel surface is the inclination angle of the two-dimensional matrix relative to the axis of shift H.
Next, the other problem is described with reference to FIG. 11. FIG. 11 schematically shows, at “a”, a front view of the system for recording (imaging) a radiographic image, which includes the stand 101, the imaging plane 102, and the grid 103, as with FIG. 10. Although the radiation source is not shown in this drawing, the radiation source is disposed to apply radiation along an exposure axis which is perpendicular to the plane of the drawing.
The quadrangular panel-shaped radiographic image detector D is orientated in the same manner as shown in FIG. 10 and is to be shifted in the direction of arrow H. Then, first and second radiographic imaging operations are carried out by applying the radiation 104 to the radiographic image detector D, which is stationary before and after being shifted.
The other problem here is that the two-dimensional matrix of the pixel sections may be inclined by an angle γ with respect to the axis of the shift, i.e., the one side of the panel, in a plane parallel to the surface of the panel (i.e., a plane parallel to the plane of the drawing) due to assembly error, or the like, of the radiographic image detector D. It should be noted that only some of the pixel sections G are illustrated in FIG. 11. In such a case, radiographic images of the grid 103 imaged through the first and second application of the radiation are distorted, as shown at “b” and “c” in FIG. 11. That is, when the first and second recorded images are joined at an area in the vicinity of the lower edge of the first image and an area in the vicinity of the upper edge of the second image, misalignment which looks like faulting is generated along the joint line.
It should be noted that, also in this case, with the radiographic image detector D being set as described above, the inclination angle γ of the two-dimensional matrix relative to one side of the panel is the inclination angle of the two-dimensional matrix relative to the axis of shift H.
For example, in a case where the radiographic image detector D has a size of 40 cm×40 cm and a distance from the radiation source to the imaging plane (SID) is 180 cm, the misalignment along the joint line between the combined images is about 0.5 mm at an end of the image when the inclination angle α is 0.31 degrees, and is about 0.5 mm at an end of the image when the inclination angle γ is 0.07 degrees, which is fairly pronounced.
The above-described problems relate to cases where the quadrangular panel-shaped radiographic image detector is shifted precisely along the axis of shift which is parallel to the surface and one side of the detector, and the two-dimensional matrix of pixel sections is inclined within such a radiographic image detector. However, even when the two-dimensional matrix of pixel sections is not inclined within radiographic image detector, i.e., even when the matrix is formed parallel to the surface and one side of the quadrangular panel-shaped radiographic image detector, the similar problems occur when the radiographic image detector itself is inclined relative to the axis of shift of the detector. FIGS. 12 and 13 illustrate such cases where the two-dimensional matrix is inclined relative to the axis of shift of the detector by the inclination angles α and γ, respectively, since the radiographic image detector itself is inclined.
Further, in the above-described cases, the inclination of the matrix remains unchanged when the radiographic image detector is shifted. However, if the radiographic image detector is gradually inclined when it is shifted, the inclination of the matrix changes along with the shift of the radiographic image detector, and the similar problems occur. FIG. 14 schematically illustrates such a situation, and schematically shows, at “a”, a front view of a system for recording (imaging) a radiographic image, which includes the stand 101, the imaging plane 102, and the grid 103, as with the system shown in FIG. 10 (this is the same for FIGS. 13 and 14). Although the radiation source is not shown in this drawing, the radiation source is disposed to apply radiation along an exposure axis which is perpendicular to the plane of the drawing.
It should be noted that, in this case, the matrix is inclined and is laterally displaced along with the shift of the radiographic image detector. This phenomenon is due to such factors that a guide mechanism for guiding the radiographic image detector being shifted has low accuracy, or that there is a relatively large clearance between a guide rod and a guide member that slides along the guide rod, for example, forming the guide mechanism.
Radiographic images of the grid 103 imaged during the first and second application of the radiation in this case are as shown at “b” and “c” in FIG. 14. Also in this case, the misalignment which looks like faulting is generated along the joint line when the first and second recorded images are joined at the area in the vicinity of the lower edge of the first image and the area in the vicinity of the upper edge of the second image.
Further, as described above, the problem of misalignment along the joint line between the images occurs not only due to the inclination of the matrix, but also due to displacement of the matrix from a predetermined position during application of the radiation. Now, the displacement is described in detail.
FIG. 15 schematically shows a situation where the displacement occurs. FIG. 15 schematically shows, at “a”, a side view of a system for recording (imaging) a radiographic image, where the numeral “100” denotes the radiation source. When images to be combined are taken, the radiographic image detector D is essentially placed, during the first and second application of the radiation, at each of predetermined positions which overlap with each other to some extent. However, if a mechanism for shifting the radiographic image detector D, for example, is aged, the radiographic image detector D may be displaced from the predetermined position in a direction which is parallel to the axis of shift H during each application of radiation. FIG. 15 shows an example in which the radiographic image detector D is displaced downward by a length Δy from the predetermined position for the second application of the radiation.
Radiographic images of the grid 103 imaged during the first and second application of the radiation in this case are as shown at “b” and “c” in FIG. 15. In this case, the images are combined with assuming that the position y0 (see the drawing) on the image taken during the first imaging operation corresponds to the upper end of the image taken during the second imaging operation. Actually, however, the upper end of the image taken during the second imaging operation is displaced by the length Δy, and thus misalignment is generated along the joint line.
Further, the above-described displacement may occur not only in the direction parallel to the axis of shift H but also in a direction perpendicular to the axis of shift H. FIG. 16 schematically shows a situation where such displacement occurs. FIG. 16 schematically shows, at “a”, a front view of a system for recording (imaging) a radiographic image. Although the radiation source is not shown in this drawing, the radiation source is disposed to apply radiation along an exposure axis which is perpendicular to the plane of the drawing.
When images to be combined are taken, the radiographic image detector D is essentially placed, during the first and second application of the radiation, at each of predetermined positions which are aligned with each other in the direction perpendicular to the axis of shift H. If, however, a mechanism for shifting the radiographic image detector D is aged or the stand 101 (more particularly, a rail for guiding the radiographic image detector D being shifted) is bent, as shown in the drawing, the radiographic image detector D may be displaced from the predetermined position in the direction perpendicular to the axis of shift H during the application of radiation. In the example shown in FIG. 16, the radiographic image detector D is displaced from the predetermined position for the second application of radiation to the right by a length of Δx.
Radiographic images of the grid 103 imaged during the first and second application of the radiation in this case are as shown at “b” and “c” in FIG. 16. In this case, the images are combined with assuming that the images taken during the first and second imaging operations are aligned with each other along the transverse direction, i.e., in the direction perpendicular to the axis of shift H. Actually, however, the image taken during the second imaging operation is displaced by the length of Δx, and thus misalignment is generated along the joint line.
The above-mentioned patent document 2 discloses a method for correcting misalignment between two images when the two images are combined (in this case, misalignment due to a difference of distances between the subject and the imaging plane), in which a grid contained in a cassette is imaged together with the subject, and the correction is carried out based on the grid image contained in each image. This method, however, cannot detect the inclination and/or displacement of the two-dimensional matrix which may occur when the above-described quadrangular panel-shaped radiographic image detector is used, and thus cannot correct misalignment between the images based on such an inclination and/or displacement.